1. Field of the Invention
The invention relates to the field of diffusing aluminum into monocrystalline silicon.
2. Prior Art
Many different methods of diffusing aluminum into monocrystalline silicon have been developed. One of the known techniques involves placing silicon wafers and either elemental aluminum or an aluminum-silicon alloy into a quartz tube which is then evacuated and sealed. The sealed tube is heated to a temperature which causes the aluminum or the aluminum-silicon alloy to give off aluminum vapor and to cause that vapor to diffuse into the silicon body. Aluminum reacts easily with quartz, a reaction which interferes with the desired diffusion. To avoid this reaction, an alumina liner may be inserted into the quartz tube. This can result in surface concentrations as high as about 10.sup.18 cm.sup.-3 as described by P. Rai-Choudhury et al. in a paper entitled "Diffusion and Incorporation of Aluminum Into Silicon" in the Journal of the Electrochemical Society for May 1977, pages 762-766. For wafers of a diameter larger than about 2 inches, the tube is usually backfilled with an inert gas such as argon to prevent the tube from collapsing as a result of the surrounding atmospheric pressure. The presence of the argon results in diffusions with low surface concentrations.
A method of utilizing aluminum vapor as a dopant source which results in diffusions with high surface concentrations is described by W. Rosnowski in the Journal of the Electrochemical Society, Volume 125, No. 6, 1978, pages 957-963. In this method, prior to the diffusion process, the tube is passivated with a mixture of Al.sub.2 O.sub.3 and Si to prevent aluminum vapor from reacting with the quartz tube during the diffusion. In this method several precautions have to be undertaken to prevent the aluminum from reacting with moisture and/or oxygen.
Selective aluminum diffusion into silicon under vacuum conditions is made difficult by problems in masking against dopant penetration where no diffusion is desired. One of the known masking materials against aluminum is silicon nitride as described in British Pat. No. 1,252,281. Other masking materials are described in U.S. Pat. Nos. 4,029,528, 4,050,967, and 4,099,997 and include very thick silicon dioxide, a thick silicon dioxide layer with a thick polycrystalline silicon layer thereover, and a glass source of dopant having a silicon nitride or thick silicon dioxide layer thereover. Each of these masking materials is only effective under some conditions.
At the present time these vacuum techniques are not readily applicable to diffusing aluminum locally to a very large depth, for example, 70-100 microns. In the 1950's and more recently (U.S. Pat. Nos. 3,909,926 and 4,040,878) several methods have been developed using free aluminum metal on the semiconductor body as the dopant source. These techniques are generally considered unsatisfactory for diffusing the base regions of transistors or the base and/or gate regions in thyristors because the aluminum alloys with the silicon which degrades the silicon surface. The poor control over surface concentration and uniformity which results from alloying limits those methods to some special applications such as diffusions for diodes as described in U.S. Pat. No. 3,909,926 and grid diffusions for thyristors as described in U.S. Pat. No. 4,040,878.
Another undesirable phenomenon which can accompany aluminum diffusion into silicon is the migration of the melted aluminum out of the desired diffusion pattern during the diffusion step. This phenomenon is caused at least in part by surface tension forces involving molten aluminum. The high volatility of the aluminum at elevated temperatures and/or reaction with the ambient can result in losses of the aluminum and makes control over the diffusion process even more difficult.